1. Field of the Invention
The present invention relates to a process for producing a styrenic polymer having a syndiotactic configuration. More particularly, it pertains to a process for producing such a styrenic polymer which is capable of modifying the molecular weight of the polymer without altering its molecular weight distribution and at the same time, of curtailing the production cost of the polymer.
2. Description of the Related Arts
In recent years, there has been proposed a process for producing a styrenic polymer having a syndiotactic configuration (hereinafter sometimes referred to as "SPS") by polymerizing a styrenic monomer by means of a catalyst comprising a transition metal compound as a primary ingredient, especially a titanium compound and an alkylaluminoxane (refer to Japanese Patent Application Laid-Open No. 187708/1987).
There has also been proposed a process for efficiently producing such a styrenic polymer (SPS) by the use of a catalyst comprising a coordination complex compound composed of an anion in which a plurality of radicals are bonded to a metal and a cation, while dispensing with an aluminoxane which is expensive and is to be used in a large amount (refer to Japanese Patent Application Laid-Open Nos. 415573/1990, 415574/1990, etc.).
In addition, there has been proposed a process for polymerizing a styrenic monomer by the use of the abovementioned catalyst comprising a transition metal compound and an alkylaluminoxane, characterized in that hydrogen is made to exist as a molecular weight modifier in the polymerizing system for the purpose of modifying the molecular weight of the styrenic polymer to be produced (refer to Japanese Patent Application Laid-Open No. 179906/1988).
However, in the case of employing a catalyst comprising a coordination complex compound composed of an anion in which a plurality of radicals are bonded to a metal and a cation in place of an aluminoxane to compensate for the expensiveness of an aluminoxane and a low yield of SPS due to the use thereof, the polymerization temperature is made higher or a large amount of an alkylaluminum as a molecular weight modifier is added to the polymerization system. Nevertheless, the aforesaid method suffers the disadvantages that the yield of the SPS to be produced is still low and the amounts of residual metals are unfavorably increased.
As a result of intensive research and investigation made by the present inventors under such circumstances, it has been found that in the case of polymerizing a styrenic monomer by the use of a transition metal compound and a coordination complex compound composed of an anion in which a plurality of radicals are bonded to a metal and an anion or an aluminoxane and, when necessary, an alkylating agent, the presence of hydrogen in the polymerization system, exerts considerable effects on both the molecular weight and the molecular weight distribution of the styrenic polymer to be produced. The present invention has been accomplished on the basis of the above-mentioned finding and information.